Bacteriophage N4-infected E. coli show rifampicin and streptolydigin resistant transcription even when the drugs are added prior to infection. All N4 transcription is rifampicin resistant and therefore independent of the host RNA polymerase. Analysis of transcription during N4 infection has revealed the presence of two new rifampicin-resistant RNA polymerizing activities. The first activity is unprecedented in that its appearance does not require transcription or translation of the phage genome after infection. This activity is virion encapsulated, injected into the cell with the DNA and transcribes viral early genes. The second activity requires the proper expression and function of two N4 genes for its appearance and is responsible for the transcription of late N4 genes. Genetic studies have demonstrated that both activities are required for viral transcription and, moreover, that the virion encapsulated RNA polymerase is required directly for DNA replication. We propose to undertake a full biochemical and genetic characterization of both transcribing activities. Related to this aim will be the obtention of a restriction endonuclease map of N4 DNA to be used to map and isolate promoters for the N4 RNA polymerases. Finally, an in vitro system, capable of supporting N4 DNA replication, will be developed to study the role of the virion-associated RNA polymerase in N4-DNA synthesis.